We have shown that supplementation with vitamins B, C, and E during pregnancy and lactation to HIV-infected women is related to lower rates of breastfeeding transmission among women in advanced stage of disease, whereas vitamin A supplementation is associated with an overall increased risk. Given that vitamin A supplementation to lactating women of unknown HIV status is becoming a widespread public health practice in many developing countries, it is urgent to identify the biological mechanisms that underlie the adverse effect of vitamin A on HIV transmission. One potential explanatory mechanism is that vitamin supplements might affect the shedding of cell-free and/or cell-associated HIV in breast milk. Preliminary analyses support this notion. We propose to examine the effect of vitamin supplements (A vs. no A and multivitamins (B, C, and E) vs. no multivitamins) administered during pregnancy and lactation to HIV-1 infected women on HIV-1 viral and proviral load in breast milk, in a sub-sample of 771 women who participated in a randomized clinical trial in Tanzania. This effect will be ascertained at three time points: delivery (n=592), 3 months after delivery (n=570), and 6 months after delivery (n=516). We will also study other mechanisms to explain the effect of vitamin supplements on breastfeeding transmission; these include 1) the effect of supplements on the risk of sub-clinical mastitis as measured by the Na/K ratio, and 2) the effect of retinoic acid and/or beta-carotene on expression of CCR5 and viral replication in vitro. Next, we propose to examine the effect of vitamin supplements on the concentration of vitamins A, B12, and E in breast milk at delivery, 3, and 6 moths thereafter, and the associations between the concentration of these vitamins in breast milk and child morbidity and mortality during the first two years. Finally, we propose to examine the associations between HIV viral and proviral load in breast milk and the risk of post-natal transmission of HIV. A significant advantage of this proposal is that it would allow us to examine various relevant research questions using data and biological samples that have been already collected and appropriately stored in the context of a large ongoing trial with high rates of follow-up and compliance. [unreadable] [unreadable]